The molecules involved in HIV-1 entry have now been identified making it possible to target this process for strategies to inhibit viral infection. The process requires two molecular entities, a coreceptor that also binds chemokines and a complex formed between the HIV-1 envelope, gp120, and the primary cell receptor, CD4. Antibodies that bind to epitopes involved in the coreceptor interaction should neutralize HIV-1 and therefore be useful for passive immunity to prevent or treat infection. In this project, the investigators will use two strategies to raise such antibodies in mice. The first will be immunization with covalently crosslinked gp120-CD4 complexes to raise monoclonal antibodies against the coreceptor binding site on the viral envelope. The investigators have already shown that these complexes elicit broadly neutralizing responses that interfere in a post-CD4 binding step. The second will be to raise antibodies specific for the viral attachment site on the coreceptors CCR5 and CxCR4. Neutralizing mAbs that block entry processes will be "humanized" by recombinant DNA methods developed by this group to provide reagents for passive immunotherapy. This approach, which allows the investigators to manipulate specific immunogens in order to obtain the desired responses, will allow them to isolate neutralizing mAbs from mice and to characterize them in less time than it would take to identify, characterize, and isolate the human counterparts.